Urea is obtained commercially by reaction of NH.sub.3 and CO.sub.2 in a reactor at elevated temperature and pressure. At urea synthesis conditions, NH.sub.3 and CO.sub.2 instantaneously and completely react stoichiometrically to form ammonium carbamate. The latter is partially converted to urea and water within 20-30 minutes of residence time in the reactor. Excess NH.sub.3 above this stoichiometric ratio is used in the reactor, in an usual NH.sub.3 to CO.sub.2 overall mole ratio from 3/1 to 6/1, for the purpose of increasing the conversion of carbamate to urea. The urea synthesis reactor effluent containing urea, water, excess free NH.sub.3 and unconverted ammonium carbamate is usually let down in pressure and heated at about 200-300 psig in a carbamate decomposer for the purpose of decomposing the unconverted ammonium carbamate to NH.sub.3 and CO.sub.2 gases and to boil off excess ammonia. The NH.sub.3 and CO.sub.2 decomposer off gas with water vapor thus recovered is absorbed in water to form an aqueous ammoniacal solution of ammonium carbamate and is recycled back into the urea reactor for total recovery.
In my U.S. Pat. Nos. 3,759,992 and 3,808,271, I have described improved processes wherein a first aqueous solution containing ammonium carbamate, ammonia, urea and water, as one formed in a urea reactor, is split into a minor portion (A) and a major portion (B). The major portion (B) is heated in an ammonium carbamate decomposer. In the decomposer, ammonium carbamate is decomposed to NH.sub.3 and CO.sub.2 gases, and at least part of the excess NH.sub.3 and water are vaporized. A resulting gas phase (C) is expelled from a resulting liquid phase (D); phase (D) contains residual ammonium carbamate and NH.sub.3 dissolved in a second aqueous urea solution. Gaseous phase (C) is then contacted countercurrently with said cooled minor portion (A), for the purpose of reducing the water vapor content of gaseous phase (C). Gaseous phase (C), thus reduced in water vapor, is contacted with fresh make up CO.sub.2 and urea other than that present in the reactor effluent, with generation of heat of reaction. Such heat of reaction is transferred indirectly either to cooling water or to a relatively colder process fluid for heat recovery.